Regional Fault Zone Research Articles

The Hospital coal basin (Massif Central, France): relay on the left-lateral strike-slip Argentat fault in relation to the Variscan postorogenic extension

Abstract Structural and microstructural analyses of the Argentat fault, combined with sedimentological and structural analyses of the associated Hospital basin allow us to discuss the tectonic control of coal basins by crustal-scale faults during the late Palaeozoic evolution of the Variscan lithosphere in the French Massif Central. The brittle Argentat fault zone consists of first- and second-order strike-slip faults, with dominant NNW-sinistral faults, NNE-dextral or sinistral faults and secondary ENE-dextral faults. Several experimental and theoretical models explain the observed fault patterns, like en echelon faults, A-type secondary faults, conjugate faults and Riedel shears. Strike-slip faulting is responsible for folding of the metamorphic formations characterized by N-S to NE-SW-trending axis. The regional-scale geometry of brittle faults and associated folds corresponds to a positive flower structure centered on the brittle Argentat fault, combined to a negative flower structure centered on the coal basin. Using tectonic inversion software, we show that these structures result from a left-lateral movement of the brittle Argentat fault in relation to a tectonic regime intermediate between extension and strike-slip, with a horizontal NE-SW to NNE-SSW-trending maximum stretching axis. Detailed map and cross-sections, and sedimentological interpretations of the late Stephanian Hospital basin show the occurrence of intra-basin syn-sedimentary strike-slip faults and progressive overlaying, indicating that sedimentation occurs during left-lateral strike-slip faulting and folding of basement along the Argentat fault. These data are consistent with a model of N-S to NE-SW-trending postorogenic extension proposed to account for the late Carboniferous evolution of the Variscan lithosphere. They also point out the complexity and the variety of structures developed along a regional brittle strike-slip fault zone and the necessity to take into account all the structures and the resulting geometry of the basement in order to better constrain the tectonic setting of intra-continental deposits.

Regional target generation for porphyry Cu–Mo systems in the Erdenet area, Mongolia

A regional structural interpretation was undertaken from an integrated study of high-resolution airborne geophysics, JERS-1/SAR and geological data around the Erdenet porphyry Cu?Mo deposit in northern Mongolia. Geophysical signatures of the Erdenet deposit have been identified and extrapolated regionally. The district-scale area was divided into five domains based on a combination of structural and geophysical character. The most prospective domain contains numerous concentric, high to low magnetically zoned bodies, that represent igneous plutons of the Selenge Complex and a series of discrete low or remanently magnetised bodies of the younger Erdenet Complex porphyries. The domains are delineated or enveloped by a series of major regional fault or shear zones, with approximate N-, W- or NW-trends. The porphyries of the Erdenet Complex appear to align along a NW-trending sub-domain boundary. Major ?late? W-trending, left-lateral transpressional structures, such as the Vitim Suture Zone, dominate the region and appear to control the distribution and orientation of the magnetic/structural domains. Application of a Riedel, simple shear ellipsoid model, suggests that NW-trending structures that appear to control the current position of the Erdenet Complex porphyries are equivalent to X or P shears. Target areas were delineated based on structural and/or litho-magnetic features. Subsequent ground verification, IP surveys and drilling have tested some of these targets and identified significant hydrothermal alteration.

Hercynian Acid Magmatism and Related Mineralizations in Northern Portugal

Cabeceiras de Basto (CB), Vieira do Minho (VM) and Vila Pouca de Aguiar (VPA) granite plutons are located in the so-called Central Iberian Zone, northern Portugal. U-Pb zircon and monazite geochronology yielded the minimum emplacement age of 311 Ma for CB and the crystallization ages of 311 Ma for VM and 299 Ma for VPA, constraining the time interval of the third Hercynian deformation phase, relative to which these massifs can be grouped as follows: syn- to late-tectonic (CB and VM) and post-tectonic (VPA) granites. CB is a composite peraluminous two-mica granite pluton, occupying the core of a N 130° E antiform. Geochemical and isotopic data suggest a heterogeneous crustal source of mid-Proterozoic age (1.2 Ga). The granites are specialized in Sn, Li and, to a lesser extent, W mineralizations. Fractional crystallization is the main concentrator mechanism for these elements. Late-magmatic hydrothermal alteration processes (feldspar albitization and plagioclase and biotite muscovitization) enhanced the concentration promoted by the magmatic differentiation. Both the VM and VPA plutons are controlled by deep regional fault zones and consist of composite biotite granites, generated in the lower crust and installed in higher structural crustal levels, resulting in thermal metamorphism (P:2 kb; T: 500–600°C) capable of generating crustal convecting fluids. They are spatially related to important Sn, W and Au mineralizations. Although they are not highly specialized, they have contributed significantly as heat sources for the ore concentrations.

FRATURAS CONTROLADORAS DA MIGRAÇÃO DE FLUIDOS NOS ARENITOS VILA VELHA, GRUPO ITARARÉ, BACIA DO PARANÁ

A deformation model is presented, which explains the distribution and type of fracturing in the Vila Velha Sandstones, outcropping in eastern border of Parana Basin, south Brazil. The studied sedimentary section consists of shallow sub aqueous sandstone lobes of the Campo Mourao Formation, Itarare Group, deposited in a periglacial environment. The study was based on detailed description of outcrops, interpretation of aerial photos and structural analysis of fractures and structural blocks. The sandstones occur as sub horizontal beds, tilted apart from large faults. Deformation bands, zones of deformation bands and slip planes are the recognized structural elements affecting sandstones, which are probably associated to a regional fault zone evolution. The fracture system is conjugated, striking mainly to N10-20E and N60-80W. Fracture induced permeability is inferred from large and small-scale structures in outcrops, where the wall rocks are oxidized and silicified close to slip planes. This situation suggests that the fractures functioned as conduits during its formation, being transformed to barriers to fluid migration after mineral precipitation.

Carbonate sedimentation in a starved pull‐apart basin, Middle to Late Devonian, southern Guilin, South China

ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle‐ to rhomb‐shaped basin filled with Devonian shallow‐ to deep‐water carbonates. This Yangshuo Basin is interpreted as a pull‐apart basin created through secondary, synthetic strike‐slip faulting induced by major NNE–SSW‐trending, sinistral strike‐slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N–S‐trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike‐slip faults; the NW–SE‐trending margins were related to parallel, oblique‐slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1–2, 3–5 and 6–9), reflecting three major phases of basin evolution.During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low‐gradient shelf. Only at the steep, fault‐controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull‐apart of the master strike‐slip faults during the late Givetian intensified the differential subsidence and produced a spindle‐shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north‐west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike‐slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform‐margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen‐depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep‐seated faults. Two ‘subdeeps’ were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian.This case history shows that the carbonate platform architecture and evolution in a pull‐apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.

Interpretation of observed fluid potential patterns in a deep sedimentary basin under tectonic compression: Hungarian Great Plain, Pannonian Basin

AbstractThe ≈ 40 000 km2 Hungarian Great Plain portion of the Pannonian Basin consists of a basin fill of 100 m to more than 7000 m thick semi‐ to unconsolidated marine, deltaic, lacustrine and fluviatile clastic sediments of Neogene age, resting on a strongly tectonized Pre‐Neogene basement of horst‐and‐graben topography of a relief in excess of 5000 m. The basement is built of a great variety of brittle rocks, including flysch, carbonates and metamorphics. The relatively continuous Endrőd Aquitard, with a permeability of less than 1 md (10−15 m2) and a depth varying between 500 and 5000 m, divides the basin's rock framework into upper and lower sequences of highly permeable rock units, whose permeabilities range from a few tens to several thousands of millidarcy. Subsurface fluid potential and flow fields were inferred from 16 192 water level and pore pressure measurements using three methods of representation: pressure–elevation profiles; hydraulic head maps; and hydraulic cross‐sections.Pressure–elevation profiles were constructed for eight areas. Typically, they start from the surface with a straight‐line segment of a hydrostatic gradient (γst = 9.8067 MPa km−1) and extend to depths of 1400–2500 m. At high surface elevations, the gradient is slightly smaller than hydrostatic, while at low elevations it is slightly greater. At greater depths, both the pressures and their vertical gradients are uniformly superhydrostatic. The transition to the overpressured depths may be gradual, with a gradient of γdyn = 10–15 MPa km−1 over a vertical distance of 400–1000 m, or abrupt, with a pressure jump of up to 10 MPa km−1 over less than 100 m and a gradient of γdyn > 20 MPa km−1.According to the hydraulic head maps for 13 100–500 m thick horizontal slices of the rock framework, the fluid potential in the near‐surface domains declines with depth beneath positive topographic features, but it increases beneath depressions. The approximate boundary between these hydraulically contrasting regions is the 100 m elevation contour line in the Duna–Tisza interfluve, and the 100–110 m contours in the Nyírség uplands. Below depths of ≈ 600 m, islets of superhydrostatic heads develop which grow in number, areal extent and height as the depth increases; hydraulic heads may exceed 3000 m locally. A hydraulic head ‘escarpment’ appears gradually in the elevation range of − 1000 to − 2800 m along an arcuate line which tracks a major regional fault zone striking NE–SW: heads drop stepwise by several hundred metres, at places 2000 m, from its north and west sides to the south and east. The escarpment forms a ‘fluid potential bank’ between a ‘fluid potential highland’ (500–2500 m) to the north and west, and a ‘fluid potential basin’ (100–500 m) to the south and east. A ‘potential island’ rises 1000 m high above this basin further south.According to four vertical hydraulic sections, groundwater flow is controlled by the topography in the upper 200–1700 m of the basin; the driving force is orientated downwards beneath the highlands and upwards beneath the lowlands. However, it is directed uniformly upwards at greater depths. The transition between the two regimes may be gradual or abrupt, as indicated by wide or dense spacing of the hydraulic head contours, respectively. Pressure ‘plumes’ or ‘ridges’ may protrude to shallow depths along faults originating in the basement. The basement horsts appear to be overpressured relative to the intervening grabens.The principal thesis of this paper is that the two main driving forces of fluid flow in the basin are gravitation, due to elevation differences of the topographic relief, and tectonic compression. The flow field is unconfined in the gravitational regime, whereas it is confined in the compressional regime. The nature and geometry of the fluid potential field between the two regimes are controlled by the sedimentary and structural features of the rock units in that domain, characterized by highly permeable and localized sedimentary windows, conductive faults and fracture zones. The transition between the two potential fields can be gradual or abrupt in the vertical, and island‐like or ridge‐like in plan view. The depth of the boundary zone can vary between 400 and 2000 m. Recharge to the gravitational regime is inferred to occur from infiltrating precipitation water, whereas that to the confined regime is from pore volume reduction due to the basement's tectonic compression.

Ore-Forming Fluids of Vein-Type Fluorite Deposits of the Cerro Aspero Batholith, Southern Cordoba Province, Argentina

Vein-type fluorite deposits in the southern part of the Sierras Pampeanas, Córdoba Province, Argentina, occur mainly hosted by calc-alkaline porphyritic biotite granites, which belong to the Paleozoic, post-tectonic Cerro Aspero batholith. The fluorite veins, of Cretaceous age, occupy steeply dipping, strike-slip regional fault zones, and are composed of fluorite and chalcedony, locally with subordinate amounts of pyrite and, in some cases, coffinite and pitchblende. These veins show typical open-space-filling textures and are closely related to pervasive silicic and argillic alteration of the host granite. Three successive stages of mineralization were distinguished on the basis of vein chronology, REE data, and fluid-inclusion study in fluorite ores. These stages generally display slightly fractionated REE patterns (La/Yb = 1.4 to 14), with REE behavior given by a relatively stronger LREE fractionation with respect to HREE. The REE composition of the fluids responsible for fluorite deposition was largely controlled by differential mobility of the REE during the silicic or argillic alteration of the host granite. Preferential leaching of HREE over LREE occurred during both alteration types, but in the argillic alteration the LREE were practically not removed. The total homogenization of primary-like aqueous inclusions took place invariably in the liquid phase at temperatures ranging from 187°C to 103°C, with concentrations of values around 160°C, 136°C, and 116°C (stages I, II, and III, respectively), defining a clear trend of fluid cooling. This cooling is accompanied by large changes in the fO2 of the fluid, from oxidizing to reducing, as inferred from the Eu/Eu∗ ratios and the mineral assemblage (pyrite, pitchblende, and coffinite). The three stages of fluorite deposition exhibit temperatures of ice melting within the interval from −0.3°C to +0.4° C, indicating that the mineralizing fluids were exclusively aqueous and highly dilute. No evidence of fluid mixing or boiling was found. The fluid-inclusion data suggest that the proposed three stages of mineralization probably were the result of a single hydrothermal event, and strongly support a single, uniform fluid reservoir for the ore-forming solutions; evidently, the latter were heated meteoric waters rather than fluids generated in deep-seated environments within the crust.

The ∼10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand:: Part 1. Eruptive processes during regional extension

A sequence of six major (each ∼1 km 3) and several minor pyroclastic eruption episodes occurred in the Tongariro Volcanic Centre (TgVC) during a period of uniquely intense activity at ∼10 ka ( 14 C years B.P.). This andesitic/dacitic sequence, named Pahoka–Mangamate (PM), had an apparent duration of ∼200 to 400 years. Rhyolite tephras erupted from Taupo are interbedded within the PM deposits. Our mapping of PM ejecta distribution has shown that multiple vents between Tongariro and Ruapehu volcanoes were active during each of the ∼10 ka episodes. The PM vents are located on a 20-km long NNE-trending linear vent zone, lying within a graben structure defined by regional fault zones. Some of these faults appear to have ruptured during the ∼10 ka eruptive sequence. The pattern of fault and vent zones suggests that an episode of accelerated extension (rifting) occurred across the TgVC at 10 ka, marked by normal displacements of the graben margin faults, and by dike intrusion beneath the vent zone on the graben axis. Juvenile ejecta from each of the PM eruptions is chemically and petrographically diverse, indicating that a number of separate magma bodies were tapped during the ∼10 ka eruption sequence (see companion paper). Despite this compositional diversity, each of the PM pyroclastic deposits is characterised by poorly vesicular juvenile lapilli with fracture-bounded surfaces, indicating interaction at depth between rapidly rising magmas and abundant groundwater. Widespread (plinian) dispersal of the PM tephras demonstrates that relatively high eruption columns were produced, despite this water-cooling of the erupting magmas. The PM eruptions and associated faulting were triggered by rifting processes. The coincident eruption of voluminous rhyolite tephras from a linear multiple vent zone in the Taupo Volcanic Centre, 50 km to NE of Tongariro, demonstrates that the 10 ka extensional episode was of regional extent.

The geology and gold deposits of the Victorian gold province

The Palaeozoic succession of Victoria represents a major world gold province with a total production of 2500 t of gold (i.e. 78 million oz). On a global scale, central Victoria represents one of the most gold mineralized areas outside the Witwatersrand of South Africa, and remains the prime example of a ‘slate belt’ gold province (also known as ‘turbidite-hosted’, or ‘shale-greywacke’ gold province). Production came from quartz veins (1000 t), modern placers (1200 t) and palaeoplacers (300 t). Up to 7000 mines produced gold from quartz veins; however, the 168 mines of this type which produced over 1 t Au contributed 68% of the primary gold production. Twelve goldfields have each produced 30 t Au (approximately one million ounces) from all sources, with Bendigo (697 t), Ballarat (408 t) and Castlemaine (173 t) being the largest: these twelve goldfields have contributed 71% of the combined primary and secondary production. Gold mining commenced in 1851, declined dramatically around 1914, and has not increased significantly since. The Palaeozoic succession of Victoria is part of an extensive fold belt along much of eastern Australia (i.e. the Tasman Orogenic Belt). In Victoria, the succession is dominated by Cambrian to Early Devonian clastic metasedimentary rocks that have undergone major upright deformation and low-grade metamorphism, culminating in the Middle Devonian (Tabberabberan) Orogeny. North-south inliers of Cambrian metavolcanic- and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones. In central Victoria, there are particularly well-developed acid volcanic complexes of Late Devonian age, and synchronous peraluminous granites which represent high-temperature crustal melting. Tertiary to Quaternary basalts have covered some of the Palaeozoic gold deposits, and have also covered several rich gold placer deposits of Cainozoic age. Larger goldfields, including Bendigo and Ballarat, occur in Ordovician slate and greywacke sequences, but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields. Most, if not all, primary deposits demonstrate structural control, although the controlling structural features differ between deposits. Wallrock alteration is strongly influenced by host rock: in metasedimentary rocks, it is limited and subtle, but it is more pervasive in mafic and felsic igneous rocks. Carbonates, muscovite and pyrite are the most widespread alteration minerals in these rocks, representing CO 2, K and S addition. Enrichment of As within gold deposits is common, with Sb important in some small deposits, whereas base metals are only abundant locally, and Bi, W, Mo and Te show strong spatial association with granites and are rarely associated with economic gold deposits. Granites are an important component of the Palaeozoic succession in Victoria. No large deposits are within granites, but a few goldfields, including Maldon (65 t Au), are within contact aureoles of either S or I type granites of differing degrees of fractionation and silica content. A single-stage, but protracted and possibly diachronous, period of gold introduction near the time of the Middle Devonian (Tabberabberan) deformation can explain geological relationships at many gold deposits. The possibility that this event was slightly older (i.e. Silurian or Early Devonian) in the west of the state cannot be precluded from the available evidence. The mineralizing event appears linked to the same thermal event as diorite (‘lamprophyre’) dykes and graite intrusions, acidic and basaltic volcanism, deformation and regional metamorphism. The limited amount of fluid chemistry data from Victorian gold deposits suggests low-salinity fluids which contained CO 2, and variable methane, with homogenization temperatures from 150 to 300°C. The sulphur isotopic compositions of pyrite are mostly +1 to +5‰, with some values slightly above + 10‰ Mineralizing temperatures are inferred to be near 300°C, and pressure around 1–2 kbar. Sulphide-rich gold deposits in the east of Victoria, many of which contain significant base metals, contrast with the rest of the province (which contains mostly gold deposits with lower sulphide content), and this eastern area should possibly be viewed as separate from the Victorian gold province, per se. The benefits to Victorian gold mining coming from the 1980's global gold boom have been insignificant compared to the historical importance of the Victorian gold province. There seems little justification for assuming that all the large deposits have been discovered, and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types. A major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival.

Geologic Evolution of a Pliocene-Pleistocene Salt-Withdrawal Minibasin: Eugene Island Block 330, Offshore Louisiana

The spatial and temporal distributions of reservoir sands are documented within the context of an evolving Pliocene-Pleistocene salt-withdrawal shelf minibasin. The Eugene Island Block 330 field, a giant oil and gas field in offshore Louisiana, is contained within the minibasin. Based on the stratigraphic and structural analyses, we present a sequence-stratigraphic and tectonic-stratigraphic model for reservoir prediction in complex shelf Gulf of Mexico minibasins. The minibasin evolved in three phases: prodelta, proximal deltaic, and fluvial. In the prodelta phase, bathyal and outer neritic shales and turbidites loaded and mobilized an underlying salt sheet. During the proximal deltaic phase, salt continued to withdraw from beneath the minibasin, and lowstand shelf margin deltas remained at a regional fault zone on the northern margin of the minibasin. Sediment accumulation and fault slip rates were high as thick sequences of deltaic sands were deposited adjacent to the fault system. During the final fluvial phase, salt withdrawal waned; consequently, the creation of accommodation space within the minibasin ceased. The basin infilled and, during lowstands, deltaic systems prograded southward. Unconformities developed in the minibasin during the e lowstands. During transgressions, thick packages of shallow-water deltaic and fluvial sands (capped by shales) were deposited on top of the unconformities. One major reservoir, the Lentic, was deposited during the prodelta phase. The hydrocarbons are trapped by deep early faults within this geopressured sand. Most of the major exploited reservoirs of the Block 330 field were deposited in the proximal deltaic phase. Reservoirs deposited in this phase are laterally extensive proximal deltaic sands that have good lateral seals because of the amount of fault activity that occurred in this interval. Only one major reservoir was deposited in the fluvial phase. This reservoir was deposited while the basin-bounding faults were still active, so the reservoir has four-way closure. Sands deposited later in the fluvial phase tend to lack lateral seals and structural closure.

Gold deposits and hydrothermal eruption breccias associated with a maar volcano at Wau, Papua New Guinea

Emplacement of endogenous domes and eruption of pyroclastic flows in the Morobe goldfield took place in the interval from 4 to 2.4 m.y. ago. During the later part of this time interval, following tilting of the ignimbrite pile, a maar was generated at Wau. The subjacent diatreme is believed to extend downward for about 1 km to a regional dip-slip fault zone which hosts deeper level gold mineralization at the nearby Ribroaster Mine. Following initial maar formation, hydrothermal brecciation and gold deposition were controlled by low-angle extensional faults which developed during incipient subsidence of a wedge of poorly supported rock between the regional fault zone and the ring fault delimiting the maar. Epiclastic sedimentation was widespread in the maar. Volatile-depleted magma reached the surface around the perimeter of the maar during active sedimentation and constructed two principal endogenous domes, one dacitic and the other andesitic. The dacitic dome, dated previously at 2.4 m.y., gave rise to an apron of crumble breccia along the northwestern edge of the maar. Accumulation of the intramaar sequence was accompanied and followed by widespread subsidence and slumping which resulted in steep dips and locally overturned beds. The regional drainage system eventually breached the crater rim and resulted in deposition of channel conglomerates and fluviolacustrine beds, all carrying alluvial gold. Radiocarbon dating of wood from this unit showed that it accumulated >42,000 years ago. The most recent events at Wau are believed to be a series of hydrothermal eruptions which constructed a composite breccia apron at Koranga Crater on the southeastern edge of the maar. The most recent eruption is tentatively inferred to have taken place in 1967.--Modified journal abstract.

Cenozoic Diapiric Traps in Eastern China: ABSTRACT

Genetically, there are 2 types of Cenozoic diapiric traps in the oil fields in eastern China. One type is produced by cold diapirism owing to the rise of evaporites and soft mudstone. This type can be divided into 3 patterns. The first pattern is the faulted ridge with 1,000 m (3,300 ft) closure and flanks dipping up to 30°. A complex graben system is developed on the top. The amplitude of the core of the anticline is about 3,000 m (9,800 ft). The Xiangzheng structure in the Shengli oil field and the Wang-cung structure in the Qian-jiang depression are examples. The second pattern is the gentle anticline or dome with 50-300 m (160-985 ft) closure and 3°-10° dip on the flanks. The incompetent strata beneath it are about 1,000 m (3,300 ft) thick. The Tuocun -Shengli structure in the Shengli oil field is an example. The third pattern is a nose-like structure with less than 50 m (160 ft) closure. This pattern is usually located near the zero edge of incompetent strata. The Serniusi structure in the Dagang oil field is an example. Another type of Cenozoic diapiric trap results from hot diapirism associated with the intrusion of gabbro or diabase. Such traps are typically small, round domes. The dip of the flanking strata generally increases with depth as the diapir is approached. A graben system is developed on top of the diapir. The distribution of these traps is related usually to regional fault zones and coincides with the distribution of the magmatism. The Matouzung structure in the Jinhu depression is one of the examples. End_of_Article - Last_Page 520------------

Heat flow in the Coso Geothermal Area, Inyo County, California

Obvious surface manifestations of an anomalous concentration of geothermal resources at the Coso geothermal area, Inyo County, California, include fumarolic activity and associated hydrothermally altered rocks. Pleistocene volcanic rocks associated with the geothermal activity include 38 rhyolite domes occupying a north trending structural and topographic ridge and numerous basaltic cinder cones and lava flows partly surrounding its southern half. In an investigation of the thermal regime of this Basin and Range geothermal area, temperature measurements were made in 25 shallow and 1 intermediate depth borehole. Geothermal gradients ranged from 25.3°C/km to 906.°C/km. The high gradients arise from convecting hot water and former convective transport of heat by dikes that fed the domes and flows. Thermal conductivity measurements were made on 312 samples from cores and drill cuttings. The resultant terrain‐corrected heat flow values range from 1.6 to 23. HFU. The actual process by which heat is transferred is rather complex; however, the heat flow determinations can be divided into two groups. The first group, less than 4.0 HFU, are indicative of regions with primarily conductive regimes, although deep‐seated mass transfer is implied. The second group, greater than 4.0 HFU, are characteristic of regions with considerable convective heat transfer in the shallow subsurface. The general shapes of the 3‐and 5‐HFU contours in the upper 35 m of the subsurface are essentially aligned with the major regional fault zones, suggesting that the high heat flows are the result of convective heat transfer caused by circulation of hot groundwater in these vertically permeable subsurface channels. The high heat flow values (>10 HFU) are essentially restricted to the rhyolite dome field and the associated surface thermal manifestations. Heat transferred by convection of water would be rapidly exhausted if it were not intermittently supplied with heat energy from depth; therefore the heat flow data substantiate the concept that the surface hydrothermal activity and associated volcanic rocks are products of a long‐lived magmatic system that has periodically erupted lava during the past 0.3–1.0 m.y. This magmatic system is the crustal heat source for the present‐day Coso geothermal system (1 HFU = 41.87 mW/m2).

Differential Compaction as a Cause of Regional Contemporaneous Faults

Regional contemporaneous faults occur on the inshore sides of major Gulf Coast depocenters and develop as normal faults with major negative displacements on the Gulf, or depocenter, side. Displacement appears to be continuous, first increasing and then decreasing in rate, during the period of rapid sediment accumulation for a particular depocenter. As new depocenters are established higher in the section and gulfward, regional contemporaneous fault zones are established on the inshore sides of the new depocenters. The distribution and characteristics of the faults suggest that they are sediment-related and are caused by large differences in compaction of adjacent sediment masses. A model was constructed to test the hypothesis that differential compaction could cause faulting in unconsolidated sediment. Mud volcanoes were produced during uniform, passive compaction, and faults were produced during forced, differential compaction of clay-water suspensions. Stress analysis indicates that the faults are simple shear failures. The stress field is approximately that of a point-supported beam with distributed loads along part of the length. Differential compaction faults may occur wherever large changes in total compaction of adjacent sediment masses occur, either as a result of change in thickness or of facies change. In the Gulf Coast area they occur on the inshore side of flexures, where both thickening and change from sand to clay facies occur in the offshore direction.

A Tentative Classification of the Main Structural Units of the Anatolian Orogenic Belt

A brief summary has been given, in five tables, of the main structural and stratigraphic units of the Anatolian orogenic belt and their possible continuation in southern Europe (the Balkans and northern Italy), based on the author's field observations in the Lombardic Alps (1929-31), Egypt (1937-40), Syria and Palestine (1943), and Turkey (1940-42) and on further evidence from the literature. The main clue to this comparative analysis of regional tectonic units in southeastern Europe and Asia Minor was obtained from a study of the boundary between the so-calledArabian and Anatolian fades in southern Turkey and the location of a regional fault zone in northern Turkey, the Izmit-Erzurum line (comparable with the San Andreas fault in California), containing the epicenters of devastating earthquakes. Stratigraphical and structural evidence indicate the continuation of this fault zone through the Struma and Morava valleys of the Balkans into the Drauzug of the Karawanken Mountains and the Insubric fault ...